1. Field of the Invention
The present invention relates to dioctyl terephthalate plasticizers, and more particularly to a method of enhancing reaction efficiency and reducing reaction time at least by 37.5% in a process for producing the dioctyl terephthalate plasticizers.
2. Description of Related Art
Dioctyl terephthalate (DOTP) is an important phthalate-free plasticizer and is environmentally friendly. DOTP plasticizers are synthesized by esterifying purified terephthalic acid (hereinafter referred to as PTA) and 2-ethylhexanol (hereinafter referred to as 2-EH). PTA is not soluble in 2-EH and normal solvents, so the esterification is a heterogeneous solid-liquid reaction, where PTA tends to have sedimentation and agglomeration. Besides, due to insolubility of PTA in 2-EH, esterification can only happen at the boundary between PTA and 2-EH, making the reaction extremely slow and time-consuming. Currently, industrial production of DOTP mainly uses titanate as the catalyst, and the reaction takes at least 5 hours. Hence, the incompatibility between PTA and 2-EH and the prolonged synthesis time have long been unsolved problems.
For improving reactivity in process for producing DOTP, a known method for catalytically synthesizing diisooctyl terephthalate uses ionic liquid of quaternary ammonium salt as a co-catalyst and a hydrotropic agent. However, since this ionic liquid may lead to side reaction or interact with catalysts, a purification step involving long standing time is required for separation, which can disadvantageously complicate and prolong the manufacturing process. US Patent Application publication 2002028963-A1 discloses a process for preparing carboxylic esters wherein water is removed by azeotropic distillation together with an alcohol. While this known approach does save some time for esterification, the decrease is only 9-11% and not significant.